Intermediate frequency amplifier



Oct. 30, 1934.

K. POSTHUMUS ET AL INTERMEDIATE FREQUENCY AMPL Filed Jan. 13, 1952 IFIER INVENTORS KLAAS POST HUMUS .WEYERS ATTORNEY Patented Oct. 30, 1934 PATENT OFFICE",

INTERMEDIATE FREQUENCY AMPLIFIER.

Klaas Posthumus and Theodorus Josephus Weyers, Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application January 13, 1932, Serial No. 586,308 In Germany June 15, 1931 2 Claims.

" mediate frequency amplifier tubes is usually connected a transformer whose primary and secondary windings are tuned by means of condensers. In this case both tuned circuits are coupled inductively so that the coupling for the higher frequencies is closer than that for the lower frequencies, due to which the resonance curve becomes unsymmetrical.

According to the invention a symmetric amplification characteristic is obtained by connecting a series of four tuned circuits between two successive tubes of the amplifier, both middle circuits being capacitively coupled together and inductively coupled with both other circuits.

The invention will be more clearly understood 4 by reference to the accompanying drawing, representing, by way of example, the intermediate frequency amplifier circuit arrangement of a superheterodyne receiver according to the invention in which only those parts of the receiver arrangement are shown which are required for a good understanding of the invention.

Both triodes V1 and V2 form part of the medium or intermediate frequency amplifier, and are coupled by a filter constituted by the four tuned circuits I, II, III and IV. These four tuned circuits are, of course, resonant to the operating intermediate frequency. The circuit I constituted by the primary winding of a transformer 2 and a condenser l, is inductively coupled with the circuit II constituted by the secondary winding of the transformer 2 and both series connected condensers 3 and 4. The condenser 4 forms at the same time part of the tuning circuit III constituted by the condensers 4 and 5 and the primary winding of a second transformer 6, so that the circuits II and III are capacitively coupled together by the condenser 4.

Finally the circuit III is inductively coupled with the circuit IV. This circuit is constituted by the secondary winding of the transformer 6 and a condenser 7. By a proper choice of the coupling factors of the transformers 2 and 6 on one hand and of the coupling condenser 4 on the other hand there may be obtained an amplification characteristic which is bilaterally symmetrical with respect to the middle of an intermediate frequency band to be transmitted from the input of tube V1 to the output of tube V2. Moreover, a proper choice of the damping decrements of the different circuits results in the amplification factor within the frequency band to be transmitted being substantially independent of the frequency. Under these circumstances the amplification characteristic resembles the most suitable form, viz, that of a rectangle.

A bilaterally symmetrical amplification characteristic is obtained when the four tuned circuits are coupled in the way as shown in the drawing; thus, when the two middle circuits are capacitively coupled, and these are coupled inductively to the other circuits. In a specific case, tried out by the applicants, the magnitudes of the capacities and inductances were as follows:

Capacity 1 and 7:160 mmF Capacity 3 and 5:170 mmF Capacity 4 28 mm]? Self-inductance of each of the coils of transformers 2 and 6:0.033 h.

Mutual inductance between the coils of each transformer=0.0019 h.

If the damping decrements in each of the circuits are too small, each of the resonance curves is too narrow, this resulting in the filter showing sags in the crest of the characteristic curve. In other words, the amplification factor within the frequency band to be transmitted is diiferent for diiferent frequencies. This fault may be removed by increasing the damping of the circuits until the amplification factor within the frequency band to be transmitted, is independent of the frequency. A further increase of the damping decrement results in an undue decrease in amplification.

In the above case best results were obtained if the resistances used in parallel with the circuits 1 and '7 were 126,600 ohms. If the internal resistances of the valves V1 and V2 are too high, a parallel resistance may be used, such that the total resistance=126,600 ohms. These values are only given by way of example. Many other dimensions of the circuits are possible.

If, for example, circuits tuned to the same frequency as the above are used, but with smaller capacity and greater inductance, the parallel resistance should be altered accordingly. Neither is it necessary to make the transformer coils equal to each other; so that it is possible to give both transformers a transformation ratio greater or smaller than one, or make that of transformer 2 greater, that of 6 smaller etc. It is, however, to be noticed that if the circuits I and IV are not equal, the parallel resistances used in these circuits should also be made unequal, such that the dampingdecrements in these circuits are again equal to each other.

While we have indicated and described one arrangement for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our invention as set forth in the appended claims.

What is claimed is:

1. An intermediate frequency amplifier circuit comprising an amplifier tube having a pair of input el ctrodes adapted to be connected to a source of high frequency energy of an intermediate frequency modulated by a band of modulation frequencies, a first resonant network including a coil and a condenser in the anode circuit of said tube, a second amplifier tube, a second resonant network including a coil and a condenser connected between the input electrodes of said last tube, a third resonant network in- "cluding a coil, coupled to said first network coil,

and a pair of condensers connected thereacross, a fourth resonant network including a coil and condenser connected across one of said pair of condensers, said last coil being coupled to said second network coil, each of said resonant networks being tuned to said intermediate frequency, and the magnitude of said one condenser and the coupling factors between said first and third network coils, and between said second and fourth network coils being so relatively proportioned that the resonance curve of said amplifier circuit is symmetrical with respect to the mid-band frequency of said modulation band.

2. An intermediate frequency amplifier circuit comprising an amplifier tube having a pair of input electrodes adapted to be connected to a source of high frequency energy of an intermediate frequency modulated by a band of modulation frequencies, a first resonant network including a coil and a condenser in the anode circuit of said tube, a second amplifier tube, a second resonant network including a coil and a condenser connected between the input electrodes of said last tube, a third resonant network including a coil, coupled to said first network coil, and a pair of condensers connected thereacross, a fourth resonant network including a coil and condenser connected across one of said pair of condensers, said last coil being coupled to said second network coil, each of said resonant networks being tuned to said intermediate frequency, and the magnitude of said one condenser and the coupling factors between said first and third network coils, and between said second and fourth network coils being so relatively proportioned that the resonance curve of said amplifier circuit is symmetrical with respect to the midband frequency of said modulation band, the damping decrements of said networks being additionally chosen to have said curve approximate a rectangle.

KLAAS POSTHUMUS. THEODORUS JOSEPHUS WEYERS. 

